The effects of testosterone on upstream migratory behavior in masu salmon,Oncorhynchus masou

Effects of testosterone (T) on upstream migratory behavior in masu salmon, Oncorhynchus masou, were studied by use of artificial raceways. In Experime nt 1, yearling precocious males castrated in August were implanted with a c apsule of medical silicone tube containing 500 mug of T in September. Their upstream migration was recorded in an artificial raceway with castrated an d sham-operated precocious males implanted with a capsule containing vehicl e for 2 months. In Experiment 2, upstream migratory behavior of yearling im mature parr implanted with a capsule containing T 500 mug or vehicle was ob served from September through November. In Experiment 3, upstream migratory behavior of castrated, castrated + T 50 mug, castrated + T 500 mug, and sh am-operated precocious males was observed from September through October. I n Experiment 4, upstream migratory behavior of the control, T 50 mug-, T 50 0 mug-, and T 1000 I cg-treated immature parr was observed from September t hrough October. In each experiment, plasma and pituitary samples were taken from the fish that moved upstream and remained to measure levels of T, gon adotropin (GTH) II, thyroxine (T,), and triiodothyronine (T,) by radioimmun oassay. Administration of T caused increases in plasma T levels within the physiological range. In Experiment 1, the frequency of migration upstream w as 0, 19.2, and 35.7% in the castrated, castrated + T 500 mug, and sham-ope rated groups, respectively. The frequency was higher in the castrated + T 5 00 mug and sham-operated groups than in the castrated group. In Experiment 2, the frequency was higher in T 500 mug-treated parr (22%) than in the con trol(2.6%). In Experiments 1 and 2, pituitary contents of GTH II in the T-t reated groups and precocious males were higher than those in the castrated precocious males and immature parr. In Experiment 3, castrated + T 50 mug, castrated + T 500 I Lg, and sham-operated fish showed upstream migratory be havior, whereas castrated fish without T did not. In Experiment 4, the freq uency was 5.7, 22.9, 17.1, and 28.6% in the control, T 50 mug-, T 500 mug-, and T 1000 mug-treated groups, respectively. In each experiment, plasma le vels of T, in migrants were lower than those in nonmigrants, whereas plasma levels of T, did not show such changes. From these results, it is inferred that T is a factor influencing upstream migration in masu salmon. (C) 2001 Academic Press.